This invention relates to quartz crystal vibrator units composed of a passive electronic element, such as a condenser or a resistor, connected to a quartz crystal vibrator element.
The various electronic parts which make up an oscillating circuit are not necessarily incorported into a single unit in such conventional devices using a quartz crystal vibrator, as, for example, a wrist watch, a clock and etc. Then it is not convenient to assemble such devices or service them because of the many terminals which need to be connected and disconnected.
Generally speaking, the center frequency of mass-produced quartz crystal vibrators are normally distributed around predetermined resonant frequency, f.sub.0. Then the oscillating frequency of each vibrator is adjusted to a predetermined oscillating frequency by incorporating therewith a passive element, for example, a variable capacitor.
A conventional oscillating circuit is shown in FIG. 1. In this figure, a quartz crystal vibrator 1 is connected in parallel with an inverter circuit 2. C.sub.G denotes a variable capacitor and C.sub.D denotes a condenser.
Deviation of the oscillating frequency of each oscillator around a predetermined center frequency is caused mainly by deviation of the natural frequency of each individual quartz crystal vibrator and partly by deviation of the value of the other associated electronic parts. Deviation of the former is far larger than that of the latter, however, the frequency adjustment of a vibrator itself is so difficult to make that the frequency adjustment of the oscillating circuit is carried out mainly by selecting appropriate condensers C.sub.G and/or C.sub.D having suitable capacitance values to achieve the frequency adjustment.
FIG. 2 shows the frequency shift of the oscillating circuit shown in FIG. 1 when either the capacitance value of condensers C.sub.G or C.sub.D is changed.
When the capacitance of condenser C.sub.G is changed, but the capacitance of condenser C.sub.D is not changed, the value of resonant frequency of the circuit is shifted along the line PQ. Likewise, if the capacitance of the condenser C.sub.D is changed and the capacitance of the condenser C.sub.G is not changed, the same effect appears. As mentioned above, the natural frequencies of mass - produced quartz crystal vibrators are normally distributed around a predetermined resonant frequency f.sub.0. This distribution is shown on the line PQ.
In order to adjust the resonant frequency of these distributed frequencies to the center frequency f.sub.0, the capacitance of both condensers C.sub.D and C.sub.G, or the value of one of them, have to be chosen. Therefore in order to precisely adjust these distributed resonant frequencies to the center frequency f.sub.0, many condensers having different capacitance values, have to be prepared. In case the natural frequencies of the mass-produced quartz crystal vibrators distribute more widely than herein described, then more condensers have to be prepared.
When a quartz crystal vibrator having a natural frequency within the range of .+-. 20 p.p.m is incorporated into the oscillating circuit, and in order to adjust its oscillating frequency within .+-. 2 p.p.m., the capacitance value of the condenser C .sub.G should be in the range of 10 - 70 pf. This has been ascertained experimentally.
When oscillating circuits are assembled, it is preferable to prepare a variable capacitor, such as a trimmer condenser, instead of preparing many fixed condensers having different capacitance values. However, it is impossible to get a small variable condenser whose variable range is from 10 pf to 70 pf. Hence the quartz crystal vibrators which are presently incorporate into watches are selected from those having only a narrow range of natural frequencies. This is the reason why the cost of an oscillator for a timepiece is expensive since its natural frequency must be extremely close to the center frequency f.sub.0 and this drawback is effectively eliminated.